Standard carburators as used on internal combustion engines have a venturi whereby the high speed air drawn into the engine sucks up fuel. The speed of the air is naturally governed by the position of a butterfly type throttle valve and, in turn, the engine speed. However when the engine is decelerating the throttle valve is almost closed and, in turn, the absolute pressure of the gases in the engine cylinders is very low, whereby the engine decelerates relatively slow.